Monday, January 22, 2007

A query at GenForum asks whether the children of half siblings may be called half cousins. (They may.) A more interesting question arises if two individuals are half cousins through both their fathers and mothers.

Let's say Bill and Bob are half brothers (they share a father, but not a mother), and that Sarah and Sue are half sisters (they share a mother, but not a father). If Bill marries Sarah and has a son Mike, and Bob marries Sue and has a son Ike, then Mike and Ike have a grandfather and a grandmother in common, thus meeting the weak definition of "full first cousins." (A strong definition of the term would require that their shared grandparents at least had dated.)

A monograph I found on Genetic and Quantitative Aspects of Genealogy confirms that single (full) first cousins and double half first cousins share the same "coefficient of relationship"—i.e. are genetically equivalent. Scroll to the bottom of the linked page for the freakiest family of all. Figure 58 explains how two individuals can have no grandparents in common, but share all eight great-grandparents. (Warning: This requires considerable wife-swapping.)